Review: Binocular stereo

1. Review: Binocular stereo • If necessary, rectify the two stereo images to transform epipolar lines into scanlines • For each pixel x in the first image • Find corresponding epipolarscanline in the right image • Examine all pixels on the scanline and pick the best match x’ • Compute disparity x-x’ and set depth(x) = B*f/(x-x’)

2. Multi-view stereo Many slides adapted from S. Seitz

3. What is stereo vision? • Generic problem formulation: given several images of the same object or scene, compute a representation of its 3D shape

4. What is stereo vision? • Generic problem formulation: given several images of the same object or scene, compute a representation of its 3D shape • “Images of the same object or scene” • Arbitrary number of images (from two to thousands) • Arbitrary camera positions (camera network or video sequence) • Calibration may be initially unknown • “Representation of 3D shape” • Depth maps • Meshes • Point clouds • Patch clouds • Volumetric models • Layered models

5. Beyond two-view stereo The third view can be used for verification

6. Multiple-baseline stereo • Pick a reference image, and slide the corresponding window along the corresponding epipolar lines of all other images, using inverse depth relative to the first image as the search parameter M. Okutomi and T. Kanade, “A Multiple-Baseline Stereo System,” IEEE Trans. on Pattern Analysis and Machine Intelligence, 15(4):353-363 (1993).

7. width of a pixel width of a pixel Multiple-baseline stereo • For larger baselines, must search larger area in second image 1/z pixel matching score 1/z

8. Multiple-baseline stereo Use the sum of SSD scores to rank matches

9. I1 I2 I10 Multiple-baseline stereo results M. Okutomi and T. Kanade, “A Multiple-Baseline Stereo System,” IEEE Trans. on Pattern Analysis and Machine Intelligence, 15(4):353-363 (1993).

10. Plane Sweep Stereo • Choose a reference view • Sweep family of planes at different depths with respect to the reference camera input image input image reference camera • Each plane defines a homography warping each input image into the reference view R. Collins. A space-sweep approach to true multi-image matching. CVPR 1996.

11. Plane Sweep Stereo • For each depth plane • For each pixel in the composite image stack, compute the variance • For each pixel, select the depth that gives the lowest variance

12. Plane Sweep Stereo • For each depth plane • For each pixel in the composite image stack, compute the variance • For each pixel, select the depth that gives the lowest variance • Can be accelerated using graphics hardware R. Yang and M. Pollefeys. Multi-Resolution Real-Time Stereo on Commodity Graphics Hardware, CVPR 2003

13. Volumetric stereo • In plane sweep stereo, the sampling of the scene depends on the reference view • We can use a voxel volume to get a view- independent representation

14. Volumetric Stereo / Voxel Coloring Discretized Scene Volume Input Images (Calibrated) • Goal: Assign RGB values to voxels in V • photo-consistent with images

15. True Scene Photo-consistency • A photo-consistent scene is a scene that exactly reproduces your input images from the same camera viewpoints • You can’t use your input cameras and images to tell the difference between a photo-consistent scene and the true scene All Scenes Photo-Consistent Scenes

16. Initialize to a volume V containing the true scene • Choose a voxel on the outside of the volume • Project to visible input images • Carve if not photo-consistent • Repeat until convergence Space Carving • Space Carving Algorithm Image 1 Image N …... K. N. Kutulakos and S. M. Seitz, A Theory of Shape by Space Carving, ICCV 1999

17. V Photo Hull Which shape do you get? • The Photo Hull is the UNION of all photo-consistent scenes in V • It is a photo-consistent scene reconstruction • Tightest possible bound on the true scene V True Scene Source: S. Seitz

18. Space Carving Results: African Violet Input Image (1 of 45) Reconstruction Reconstruction Reconstruction Source: S. Seitz

19. Space Carving Results: Hand Input Image (1 of 100) Views of Reconstruction

20. Reconstruction from Silhouettes • The case of binary images: a voxel is photo-consistent if it lies inside the object’s silhouette in all views Binary Images

21. Reconstruction from Silhouettes • The case of binary images: a voxel is photo-consistent if it lies inside the object’s silhouette in all views Binary Images • Finding the silhouette-consistent shape (visual hull): • Backproject each silhouette • Intersect backprojected volumes

22. Volume intersection B. Baumgart, Geometric Modeling for Computer Vision, Stanford Artificial Intelligence Laboratory, Memo no. AIM-249, Stanford University, October 1974.

23. Photo-consistency vs. silhouette-consistency True Scene Photo Hull Visual Hull

24. Carved visual hulls • The visual hull is a good starting point for optimizing photo-consistency • Easy to compute • Tight outer boundary of the object • Parts of the visual hull (rims) already lie on the surface and are already photo-consistent Yasutaka Furukawa and Jean Ponce, Carved Visual Hulls for Image-Based Modeling, ECCV 2006.

25. Carved visual hulls • Compute visual hull • Use dynamic programming to find rims (photo-consistent parts of visual hull) • Carve the visual hull to optimize photo-consistency keeping the rims fixed Yasutaka Furukawa and Jean Ponce, Carved Visual Hulls for Image-Based Modeling, ECCV 2006.

26. From feature matching to dense stereo • Extract features • Get a sparse set of initial matches • Iteratively expand matches to nearby locations • Use visibility constraints to filter out false matches • Perform surface reconstruction Yasutaka Furukawa and Jean Ponce, Accurate, Dense, and Robust Multi-View Stereopsis, CVPR 2007.

27. From feature matching to dense stereo • http://www.cs.washington.edu/homes/furukawa/gallery/ Yasutaka Furukawa and Jean Ponce, Accurate, Dense, and Robust Multi-View Stereopsis, CVPR 2007.

28. Stereo from community photo collections • Up to now, we’ve always assumed that camera calibration is known • For photos taken from the Internet, we need structure from motion techniques to reconstruct both camera positions and 3D points

29. Towards Internet-Scale Multi-View Stereo • YouTube video, high-quality video Yasutaka Furukawa, Brian Curless, Steven M. Seitz and Richard Szeliski, Towards Internet-scale Multi-view Stereo,CVPR 2010.

30. Fast stereo for Internet photo collections • Start with a cluster of registered views • Obtain a depth map for every view using plane sweeping stereo with normalized cross-correlation Frahm et al., “Building Rome on a Cloudless Day,” ECCV 2010.

31. Plane sweeping stereo • Need to register individual depth maps into a single 3D model • Problem: depth maps are very noisy far near Frahm et al., “Building Rome on a Cloudless Day,” ECCV 2010.

32. Robust stereo fusion using a heightmap • Enforces vertical facades • One continuous surface, no holes • Fast to compute, low memory complexity David Gallup, Marc Pollefeys, Jan-Michael Frahm, “3D Reconstruction using an n-LayerHeightmap”, DAGM 2010

33. Results • YouTube Video Frahm et al., “Building Rome on a Cloudless Day,” ECCV 2010.

34. Kinect: Structured infrared light http://bbzippo.wordpress.com/2010/11/28/kinect-in-infrared/

35. Kinect Fusion Paper link (ACM Symposium on User Interface Software and Technology, October 2011) YouTube Video